In recent years, water systems operated with high concentration by decreasing the amount of water blow-down to the outside of the system have been increasingly employed in order to reduce energy cost. Water in such water systems contains scale components including calcium, magnesium, silica, metal oxides or the like at high concentrations. Therefore, these components are deposited in a form of scale, which sometimes decreases thermal efficiency of heat exchangers and causes plugging.
In a boiler water system, scale components such as calcium, magnesium, silica, and iron brought into a boiler can form scale and are deposited on a heating surface with a high heat load. Those scale components have low thermal conductivity. Therefore, such scale causes tube failures by bulging, and burst due to overheating of boiler tubes made of steel.
Deposition of scale on a heating surface causes loss of heat transfer and decreases thermal efficiency of a boiler, which increases a fuel cost of a boiler. In a boiler water system or the like, calcium and magnesium, which are hardness components in raw water, are therefore removed with a water softener in order to prevent scale deposition, and softened water is used as feed water.
A water treatment method is also employed in which the deposition of scale components such as a trace amount of hardness components and silica in feed water, which are brought into a boiler can, in a system is suppressed by adding a scale inhibitor to boiler water, and these components are discharged to the outside of the system by performing blow-down.
A scale inhibitor inhibits formation of scale from hardness components brought into a water system. Examples of the scale inhibitor used include phosphates such as trisodium phosphate and sodium tripolyphosphate and polymers such as sodium polyacrylate.
Even when such a scale inhibition method is employed, for example, unexpected leakage of hardness components occurs in a feed water line, and scale may be deposited inside a boiler can. In this case, the boiler operation is stopped, the whole boiler water is discharged by performing blow-down, and then chemical cleaning is performed using a scale removing agent. Patent Literature 1 describes a method for removing scale by performing chemical cleaning that uses a chelating agent such as high-concentration ethylenediaminetetraacetic acid (EDTA) or an organic acid such as sulfamic acid.
The method for removing scale described in Patent Literature 1 poses problems in that the productivity is decreased because the boiler is temporarily stopped, and the cleaning cost is additionally required.
Patent Literature 2 proposes a method for removing scale without stopping the boiler operation. In the method proposed described in Patent Literature 2, a particular chelating agent such as EDTA, nitrilotriacetic acid (NTA), or diethylenetriamine and a particular disperser such as polymaleic acid are added to the boiler water, and scale is removed while the boiler is operated.
The chelating agent used in the method for removing scale in Patent Literature 2 acts on iron, which is a base material for the boiler, and thus corrosion occurs.
Patent Literature 3 proposes a method that uses a chelating agent and a corrosion inhibitor in a combined manner. In the method proposed in Patent Literature 3, the corrosion is suppressed by using a chelating agent and an aldonic acid or a salt thereof while at the same time scale is removed by using a chelating agent.
The method for removing scale in Patent Literature 3 poses problems in that the addition of the corrosion inhibitor is required in accordance with the amount of the chelating agent added, and the material cost for the corrosion inhibitor is additionally required.
Patent Literature 4 proposes a method for removing hardness scale without using a chelating agent. The method proposed in Patent Literature 4 uses a composition consisting of a mixture of a water-soluble anionic vinyl polymer sequestering agent and another water-soluble anionic vinyl polymer disperser. The agent contains at least 30 wt % of a carboxyl chelate functional unit. The agent has a chelation value of at least 200, and a molecular weight in the range of 500 to 50,000. It is described in Patent Literature 4 that when hardness components and iron are simultaneously brought into a water system, the addition of the sequestering agent and the disperser in high concentrations prevents the deposition of hardness scale and produces an effect of removing deposited scale.
However, an extremely large amount of chemicals need to be added to produce the effect of removing scale using the composition in Patent Literature 4, which poses a problem in terms of cost. Furthermore, when feed water contains hardness components at a low concentration and iron at a high concentration, the effect of removing scale decreases over time during operation, which poses a problem in that scale is not sufficiently removed.
Patent Literature 1: Japanese Patent Publication H4-193971 A
Patent Literature 2: Japanese Patent Publication 2000-154996 A
Patent Literature 3: Japanese Patent Publication 2011-212591 A
Patent Literature 4: Japanese Patent Publication S63-65999 A